In order to use a botanical resource as an alternative industrial raw material of petroleum oil effectively, the botanical resource must be used as a molecule material. Thus, it is important to efficiently separate the respective constituents of a plant that are highly combined at the molecular level while maintaining the molecular functions.
For example, wood is a composite of carbohydrates and lignin whose structure and property are completely different. As a method for separating wood into two components of carbohydrates and lignin, an organosolv method and a solvolysis method for example have been suggested. As a pre-treatment method, an explosion method and an autohydrolysis method for example have been suggested. However, in the case of the component separation by these methods, high energy is required and the separation is promoted incompletely. The reason is that cell walls include therein carbohydrates and lignin mixed in a complicated manner. Furthermore, the above separation methods cause a significant modification of lignin during an addition of high energy, thus causing a difficulty of the subsequent use.
Thus, in order to achieve a complete component separation without damaging the original characteristics of lignin, it is required to set an optimal condition for the individual constituting materials to decompose the mixed carbohydrates and lignin under a low-energy condition.
One method for breaking the wood tissue structure on the basis of the molecular level is a treatment by sulfate. For example, a concentrated sulfate treatment causes cellulose to be swollen and the cellulose is further caused to have a partial hydrolysis and dissolution, resulting in a broken cell wall structure. The wood hydrolysis method using concentrated sulfate is already substantially technically established and is a complete and low-cost method from the viewpoint of the component separation.
However, the wood hydrolysis method using concentrated acid is disadvantageous, in order to realize a component separation method for the full use of wood, in that condensed lignin is inactivated. Such highly-condensed lignin includes rigid molecules. Thus, activation by structure modification or the depolymerization thereof is difficult. This is one of the reasons that have prevented a wood processing industry using acid hydrolysis as a core. The inactivated lignin in the concentrated acid treatment process is caused by the non-existence of medium to lignin in the reaction system.
There is a technique to separate cell wall constituent that is a main constituent of a plant (i.e., a lignocellulosic composite of lignin and material such as cellulose or hemicelluloses) using phenol and concentrated acid to thereby obtain the derivative thereof (e.g., see Patent documents 1 and 2).
A phenol derivative such as cresol is a good solvent to lignin and has reactivity similar to that of a lignin aromatic nucleus. Cresol has a high affinity to lignin and always exists together with lignin in the reaction system. However, cresol cannot be substantially mixed with concentrated acid. To solve this, wood meals are firstly treated by cresol so that the interior of the wood meals is sufficiently permeated with cresol. Thereafter, concentrated acid having a high affinity to carbohydrates is added to the mixture while intensely agitating the mixture at room temperature. Then, cellulose is immediately swollen and the wood meals have a broken tissue structure, thus cellulose is hydrolyzed. Since lignin is surrounded by cresol that is not mixed with concentrated acid, at the timing of the addition of concentrated acid, the contact of lignin with acid is suppressed as much as possible. The agitation causes acid to be mixed in cresol and phenol is bound to the benzyl position of lignin, thereby generating a lignophenol derivative.
A lignophenol derivative is included in cresol solution. A cellulose hydrolysate is included in concentrated acid solution. By distilling cresol away, such a lignophenol derivative is obtained that has a lighter color and is more active than in the case of Milled Wood Lignin (MWL). On the other hand, acid solution includes therein carbohydrate as a monosaccharide such as glucose, oligosaccharide, or polymer.
During a treatment process, lignin is partially depolymerized but carbonium ion generated during the process is immediately stabilized by cresol, thus suppressing the self-condensation of lignin. After the treatment, the agitation is stopped to thereby cause a reaction mixture to be immediately separated to cresol solution and concentrated acid solution.
In the technique as described above, a method called phase separation system conversion is used in order to completely separate lignin and cellulose. Specifically, a lignocellulosic composite material is solvated by a phenol compound in advance and then lignocellulosic material is caused to have a contact with acid. As a result, lignin is caused to selectively graft the phenol compound to an acid and cellulose is swollen by an acid, and cellulose is hydrolyzed to separate lignin from cellulose. These methods also include a technique for improving the separation efficiency (Patent Document 3).
Furthermore, Patent Document 2 discloses that lignophenol is used as a component of a compact. According to this disclosure, even after the use of this compact, lignophenol can be collected again by an organic solvent and can be reused because lignophenol can be dissolved in the organic solvent.